In the processing of rolled materials, e.g. steel or various metals, which are in the form of so-called slabs or billets, the material being rolled passes along a rolling line with several rolling sections. The sections for example include a roughing, an intermediate and a finishing section. In turn, each of the rolling sections incorporates several roll stands, in which the material to be rolled is rolled out in several passes to form strips or wire. In the case of wire rolling, the roll stands used have rolls, each having two calibrated rolling rings, one above another, with bores which are alternately round and oval in shape, the cross-section of which decreases after each pass. Thus, from billets with an approximately square cross-section, round wire is manufactured.
In order to roll the material which is being rolled down to the desired cross-section at each roll stand, the rotational speeds of the rolls of the individual roll stands must be regulated to a set rotational speed value. The individual set rotational speed values, and hence also the ratios to each other of the rotational speeds of the rolls in the successive roll stands, are generally prescribed in a pass plan. In order to obtain, at the end of the rolling line, the desired thickness or the desired cross-section of the processed rolled material, the prescribed ratios of rotational speeds must be adhered to as closely as possible during the processing, i.e. the actual values of the rotational speeds of the rolls in the successive roll stands must also at all times correspond to the prescribed rotational speed ratio.
In particular during wire rolling, very high roll speeds arise, so that the pass offtakes and the ratios of the rotational speeds of the rolls in the individual roll stands must be precisely matched to each other and kept constant in order to avoid tension and compression loads on the wire between the roll stands. Even small deviations can lead to a breakage of the wire or to loop formation.
One possibility for ensuring that the rotational speed ratio is kept constant is to couple all the roll stands in a rolling section together rigidly via a mechanical transfer gearbox and to drive them by a common large motor. However, a major disadvantage of this is that, for example, if there is wear in the individual rolling rings, it is always necessary to exchange or to regrind all the rolling rings, because the cross-sections or diameters of the bores must be matched to one another in order to achieve a desired result from the rolling. This makes such an approach very time-consuming and costly.
These disadvantages can be overcome in that each roll stand of the rolling section is driven by a separate drive, that is with its own motor and gearbox. This permits the rolling speeds to be set for the rolls in each roll stand independently of each other by means of set rotational speed values for the individual drives, which can be adjusted by means of a rotational speed regulator provided for each drive. Wear of individual rolls or pairs of rolling rings can then be balanced out by a change in the set rotational speed value, in order to achieve the required rolling speed of the drive concerned. In addition, there is no need for a mechanical transfer gearbox, the design of which is demanding.
However, a major challenge of such a drive solution is presented by the regulation of the rotational speed of the individual roll stands during the processing of goods which are being rolled. When a roll stand is broached, i.e. when the goods being rolled first enter the rolls, a real load moment is imposed on them which effects a drop in the rotational speed of the rolls in this roll stand. On the other hand the rolls in the other roll stands, on which at the time point of the broaching there is no real load moment or a different one, e.g. a smaller one, have an unchanged or only slightly altered rotational speed. The consequence of this is that the rotational speeds of the individual drives or rolls are no longer working in synchrony, that is no longer in a prescribed rotational speed ratio to one another. This can lead to tension or compression loads, and hence a breakage of the wire, or to the material being rolled forming loops between the individual roll stands.
There is a known method as to how the rotational speeds of all the stands within a finishing section can be kept constant relative to one another after the imposition of real load moments, arising when material enters into the rolling gap, and tension or compression loads on the wire are thus avoided.
However, in particular if loop formation between the intermediate and finishing sections must not be permitted to occur, i.e. if the rotational speeds of the stands in the finishing section must also remain constant relative to the rotational speed of the last stand in the intermediate section, the method cited above is not adequate. Material entering into the first stand of the finishing section would, due to the real load moment, produce a drop in the rotational speed of the first stand, and the existing prescription would then ensure that the rotational speeds of the remaining stands in the finishing section would remain in step with this drop in rotational speed, that is they would remain synchronous. By comparison with the last stand in the intermediate section however, an impermissible asynchronicity in the rotational speeds would result. The existing method must therefore be supplemented by a further method in accordance with the invention.
From DE 197 26 586 A1, a method and equipment are known for the purpose of reducing or compensating drops in the rotational speed when goods which are being rolled are threaded into a roll stand. In this method, the rotational speed of the rolls in the roll stand are regulated by a regulator wherein, independently of its input, the regulator issues a predefined supplementary value in a prescribed transitional time interval, shortly before the material being rolled is threaded into the roll stand, when it is being threaded in, or shortly after it has been threaded in.
In DE 24 13 492 A1, a method and a circuit arrangement are described for the mutual matching of the drive rotational speeds in a multi-stand rolling line with individual drives. In this method for the mutual matching of the rotational speeds of the drives in successive stands of a continuous rolling line with individual drives, each of which is regulated to a set rotational speed value by a rotational speed regulation loop with an underlying moment regulation loop, when a stand is broached the change in the drive moment at the preceding stand is detected, the rotational speed regulation loop of the stand which is being broached is isolated and the underlying moment regulation loop is switched over to an artificial moment setpoint value which is formed from a prescribed value and a correction value derived from the change in the drive moment at the preceding stand. After a predefined time span—at the latest when the next-following stand is broached—the instantaneous value of the drive rotational speed is issued to the rotational speed regulation loop as an improved rotational speed setpoint value, and the rotational speed regulation loop is smoothly re-engaged.